Field of the Present Disclosure
The present disclosure relates to a delayed fluorescence material with long-time emission, and an organic light emitting device having the delayed fluorescence material.
Discussion of Related Art
To commercialize an organic light emitting device, a light emitting material should have an improved emission efficiency. For this, phosphorescence and delayed fluorescence materials have been studied actively. However, although the phosphorescence material achieves a high emission efficiency, the phosphorescence should employ a metal complex which is expensive and has a shot life-span.
As for the delayed fluorescence material, articles published in Nature (2012, 492, 234) and JACS (2012, 134, 14706) discloses TADF (Thermally Activated Delayed Fluorescence) to achieve high efficient green fluorescence materials with high external quantum efficiency. The TADF material is a substance that can up-convert a triplet excited state into a singlet excited state using a little thermal energy and efficiently exhibits light emission (fluorescence) from the singlet excited state. The fluorescence in this case is light emission from the excited singlet and hence is light emission at the same wavelength as fluorescence. However, the fluorescence has a longer lifetime of light to be emitted, i.e., a longer emission lifetime than those of normal fluorescence and phosphorescence by virtue of reverse intersystem crossing from an excited triplet state to an excited singlet state, and hence is observed as fluorescence delayed as compared to the normal fluorescence and phosphorescence. This can be defined as delayed fluorescence. In this connection, using a molecular structure including a combination of a donor unit to donate an electron and an acceptor unit to accept an electron, a small difference between the singlet and triplet excited energy states may be achieved to reach the efficient delayed fluorescence material. Since the delayed fluorescence material employs both the fluorescence light emitting and phosphorescence light emitting mechanisms, the delayed fluorescence material may remove the shortcoming of the existing fluorescence material; that is, the shortcoming in terms of the external quantum efficiency is eliminated and, further, it dispenses with the metal complex which is expensive and has a shot life-span.